In general, a magnetic recording medium such as a magnetic tape is produced by coating and drying a magnetic paint comprising a ferromagnetic powder and a binder on a nonmagnetic support.
In a magnetic recording medium, particularly a magnetic recording medium for video use which requires a high-frequency recording, there has lately been a demand for increasing the recording density. With the demand for higher-density recording, a further finer particulate and high-coercive ferromagnetic powder having a specific surface area of 55 m.sup.2 /g has come into use.
However, as the ferromagnetic powder becomes finer particulate and highly coercive, its individual particles are liable to strongly aggregate, which makes it difficult to provide any sufficient dispersibility and surface smoothness necessary for obtaining a high reproduction output and good S/N ratio in the high frequency recording. This tendency is remarkable particularly in a ferromagnetic metal powder. In a magnetic recording medium containing a ferromagnetic powder not sufficiently dispersed therein, the magnetic powder contained in its layer is liable to shed therefrom to thereby cause an undesirable phenomenon such as clogging of a magnetic head, which deteriorates its running durability.
To improve such a magnetic recording medium by getting rid of the above shortcomings, Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) No. 146433/1987 discloses a technique to improve the dispersibility of a ferromagnetic powder by incorporating into a magnetic layer both a resin comprising a repetitive unit containing a polar group and having a glass transition point of not less than 50.degree. C and a resin comprising a repetitive unit containing a polar group and having a glass transition point of not more than 50.degree. C, and also to improve the running durability by using both a fatty acid ester lubricant and a non-fatty acid ester lubricant in combination.
As a result of the investigation by the inventors, however, it has been found that, where a ferromagnetic powder is made into so fine particles as to have an increased specific surface area of not less than 55 m.sup.2 /g, even though a magnetic layer contains a resin comprising a repetitive unit containing a polar group and having a glass transition point of not less than 50.degree. C. and a resin comprising a repetitive unit containing a polar group and having a glass transition point of not more than 50.degree. C., if the difference between the glass transition points is not sufficiently large, neither a sufficient dispersibility of the magnetic powder required for high-frequency recording nor a sufficient surface smoothness of the magnetic layer can be obtained.